bugfix: read the docs (#546)

* Create .readthedocs.yml

* Create doc-requirements.txt

.readthedocs.yml

+# .readthedocs.yml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Build documentation in the docs/ directory with Sphinx
+# sphinx:
+#   configuration: docs/conf.py
+
+# Build documentation with MkDocs
+mkdocs:
+ configuration: mkdocs.yml
+
+# Optionally build your docs in additional formats such as PDF and ePub
+formats: all
+
+# Optionally set the version of Python and requirements required to build your docs
+python:
+  version: 3.7
+  install:
+    - requirements: docs-requirements.txt

doc-requirements.txt

+mkdocs>=1.1,<2
+mkdocs-material
+mdx_truly_sane_lists